Potable water reservoirs must be covered to prevent contamination and loss by evaporation. Hardshell roofs are expensive and time consuming to build. Such roofs must be designed to prevent intrusion of surface water to the reservoir water. Further, in colder climes, the roof must be designed to support snow and ice loads.
An alternative to the rigid roof system for reservoirs was presented by U.S. Pat. No. 3,313,443 which taught the use of a large flexible cover supported by floats except at the border of the reservoir. Rain water was collected in border sumps which migrated horizontally as the level of the reservoir changed. The sumps were not well defined and often developed into two or more folded channels. The float system is of course expensive and must be serviced periodically. Border sumps on large reservoirs are too far from the center and such covers often are subject to large puddles which freeze in winter or cause excessive strain in the cover.